The invention relates to a method for protection of an electrical power line, particularly in the event of short circuits, as claimed in the precharacterizing clause of claim 1. Methods such as these are used to switch off an electrical power line when a short circuit occurs, that is to say when the impedance to ground or to other phases is unacceptably low. The expression electrical power lines in this case includes, in particular, busbars, heavy-current power lines, crosscountry lines or other current-carrying parts.
In conventional methods for protection of an electrical power line, an adaptive estimator is used to determine an impedance value between the line and ground or another phase, from a number of measurement points of the current and voltage. The adaptive estimator in this case measures a number of data points of the current and of the voltage in a time measurement window, and uses their profile to calculate the real and imaginary parts of the impedance. This checks whether the impedance value is in a forbidden area, referred to as the tripping area. If this is the case, the line is switched off.
For protection of busbars using such methods, for example with distance protection, signal comparison methods and what is referred to as backward interlocking, detection times in the range from 40 to 60 ms can be achieved depending on the equipment and the fault.
The invention is based on the object of providing a method of the type mentioned initially, so that rapid detection times are ensured without any major complexity.
This object is achieved by the method as claimed in claim 1.
Thus, according to the invention, a warning signal is produced which is derived from a typical indicator of the occurrence of a short circuit. When this warning signal occurs, the measurement window is restarted and/or is shortened, so that a first estimated value of the impedance is produced as quickly as possible. This makes it possible to react to short-circuit situations more quickly.
The warning signal can be initiated when a radio-frequency signal which is above a predetermined threshold value is detected on the line. Such a signal is produced by a leader, that is to say a current pulse which generally has a duration of several hundred nanoseconds and a magnitude of several amperes. In addition to this, the warning signal can be made dependent on other criteria, for example on whether successive measurement points are more than a predetermined sudden-change value apart from one another, or whether the model used by the estimator indicates an inconsistency in the measured values.
Since the warning signal offers information about the presence of a short circuit, it can also be taken into account in the decision as to whether the circuit breaker should be interrupted.